Anurag Shyam

Influence of electrode and insulator materials on the neutron emission in a low energy plasma focus device

Insertion of internal impurities is one of the factors for the degradation of neutron output in plasma focus devices. In this context, neutron emission measurements were carried out in a 2.2 kJ (7.2 /spl mu/F, 25 kV) Mather type squirrel cage plasma focus device with various combinations of electrode and insulator materials. The neutron yield was measured by silver activation detector. The results indicate that a central electrode of low erosion rate material produces maximum neutron yield with the highest anisotropy factor. A time integrated X-ray pinhole image shows that focus corresponding to the highest neutron yield material is the most compressed. The dielectric constant of the ceramic insulators has, at best, a very weak correlation with neutron output. The nonceramic insulators like perspex, nylon, or teflon neither produce focus nor neutrons.

Development of compact size penning ion source for compact neutron generator

For long-life operation, easy to mount and compact in size penning type ion sources are widely used in different fields of research such as neutron generators, material research, and surface etching. One penning type ion source has been developed in our laboratory. Applying high voltage of 2 kV between two oppositely biased electrodes and using permanent magnet of 500 gauss magnetic field along the axis, we had produced the glow discharge in the plasma region. The performance of this source was investigated using nitrogen gas. Deuterium ions were produced and extracted on the basis of chosen electrodes and the angle of extraction. Using a single aperture plasma electrode, the beam was extracted along the axial direction. The geometry of plasma electrode is an important factor for the efficient extraction of the ions from the plasma ion source. The extracted ion current depends upon the shape of the plasma meniscus. A concave shaped plasma meniscus produces converged ion beam. The convergence of extracted ions is related to the extraction electrode angle. The greater the angle, the more the beam converges. We had studied experimentally this effect with a compact size penning ion source. The detailed comparison among the different extraction geometry and different electrode angle are discussed in this paper.

Effect of anode and insulator materials on plasma focus sheath (pinch) current

Plasma sheath (pinch) current in a low energy (2.2 kJ) plasma focus device was measured fur different anode and insulator materials. Among the anode materials, the highest sheath current was observed with tungsten and the lowest with aluminum. Among the ceramic insulators the maximum plasma sheath current was obtained with quartz and the minimum with alumina. The computed high Z (atomic number) impurities in plasma sheath, however, were least in the plasma focus with alumina insulator. None of the nonceramic (plastic) insulators produced neutrons, as the plasma sheath was nonuniform and was highly contaminated with impurities.

Neutron Emission Characteristics of NX-3 Plasma Focus Device: Speed Factor as the Guiding Rule for Yield Optimization

This paper reports the results of characterization and optimization experiments carried out on a newly developed NX-3 dense plasma focus device (20 kJ at 20 kV, a quarter time period of ~ 3 μs, and 10 kJ/600 kA at 14 kV) at the Plasma Radiation Source Laboratory, NIE, Nanyang Technological University, Singapore. Initial experiments were conducted with an electrode assembly configuration having anode radius and length of 20 and 160 mm, respectively, for detailed neutron emission characterization of NX-3 device followed by further optimization of neutron yield using various other electrode configurations designed using the Lee Code. At ≥10-kJ operation, the average neutron yield on the order of 109 neutrons/pulse in 4πsr was obtained for the deuterium filling gas pressure range of 6-8 mbar. The neutron yield of ~ 4.6 ×109 neutrons/pulse at 10 kJ/6 mbar is the highest ever reported for a device with the same stored energy. The neutron anisotropy measurements point to the beam-target mechanism as the dominant neutron production mechanism for NX-3 plasma focus device. Further optimization of neutron yield in NX-3 was achieved with the peak average neutron yield being enhanced from ~ (2.38 ±0.31) ×109 neutrons/shot for the initial electrode configuration to about ~ (3.40 ±0.43) ×109 neutrons/shot for the electrode configuration with anode radius and length of 26 and 140 mm, respectively. The analysis of neutron yield results for various electrode assembly configurations demonstrates the speed factor as a key optimization tool for maximization of neutron yield.

Note: Compact helical pulse forming line for the generation of longer duration rectangular pulse

The helical pulsed forming line (PFL) can generate longer duration rectangular pulse in a smaller length. A compact PFL using helical water line is designed and experimentally investigated. The impedance of the helical PFL is 22 [ohm sign]. The compactness is achieved in terms of reduction in length of the PFL by a factor of 5.5 using helical water PFL as compared to coaxial water PFL of same length. The helical PFL was pulsed charged to 200 kV using a high voltage pulse transformer in 4.5 μs and discharged into the matched 22 Ω resistive load through a self-breakdown pressurized spark gap switch. The rectangular voltage pulse of 100 kV, 260 ns (FWHM) is measured across the load. The effect of reduction in water temperature on the pulse width is also studied experimentally. The increase in pulse width up to 7% more is observed by reducing the temperature of the deionized water to 5 °C. It will further reduce the length of the PFL and make the system small for compact pulsed power drivers.

Compact pulse forming line using barium titanate ceramic material

Ceramic material has very high relative permittivity, so compact pulse forming line can be made using these materials. Barium titanate (BaTiO(3)) has a relative permittivity of 1200 so it is used for making compact pulse forming line (PFL). Barium titanate also has piezoelectric effects so it cracks during high voltages discharges due to stresses developed in it. Barium titanate is mixed with rubber which absorbs the piezoelectric stresses when the PFL is charged and regain its original shape after the discharge. A composite mixture of barium titanate with the neoprene rubber is prepared. The relative permittivity of the composite mixture is measured to be 85. A coaxial pulse forming line of inner diameter 120 mm, outer diameter 240 mm, and length 350 mm is made and the composite mixture of barium titanate and neoprene rubber is filled between the inner and outer cylinders. The PFL is charged up to 120 kV and discharged into 5 Ω load. The voltage pulse of 70 kV, 21 ns is measured across the load. The conventional PFL is made up of oil or plastics dielectrics with the relative permittivity of 2-10 [D. R. Linde, CRC Handbook of Chemistry and Physics, 90th ed. (CRC, 2009); Xia et al., Rev. Sci. Instrum. 79, 086113 (2008); Yang et al., Rev. Sci. Instrum. 81, 43303 (2010)], which increases the length of PFL. We have reported the compactness in length achieved due to increase in relative permittivity of composite mixture by adding barium titanate in neoprene rubber.

Low voltage operation of plasma focus

Plasma foci of compact sizes and operating with low energies (from tens of joules to few hundred joules) have found application in recent years and have attracted plasma-physics scientists and engineers for research in this direction. We are presenting a low energy and miniature plasma focus which operates from a capacitor bank of 8.4 muF capacity, charged at 4.2-4.3 kV and delivering approximately 52 kA peak current at approximately 60 nH calculated circuit inductance. The total circuit inductance includes the plasma focus inductance. The reported plasma focus operates at the lowest voltage among all reported plasma foci so far. Moreover the cost of capacitor bank used for plasma focus is nearly 20 U.S. dollars making it very cheap. At low voltage operation of plasma focus, the initial breakdown mechanism becomes important for operation of plasma focus. The quartz glass tube is used as insulator and breakdown initiation is done on its surface. The total energy of the plasma focus is approximately 75 J. The plasma focus system is made compact and the switching of capacitor bank energy is done by manual operating switch. The focus is operated with hydrogen and deuterium filled at 1-2 mbar.

Neutron emission from a 100 joule plasma focus

A neutron yield of } 5×105D-D neutrons/discharge was obtained when 0.1 KJ of energy was pumped into a plasma focus of Mather type. The neutron yield was measured by a high sensitivity silver activation counter. The results are compared with those of other laboratories and it is found that neutron yield scales asE1.73 orI4.29 in the 0.1–500 KJ region.

Effect of electrode materials on the neutron emission from a plasma focus

Abstract Experiments were conducted on a plasma focus to determine the electrode material that will enhance the neutron yield from the device. The focus was studied using electrodes of Al, Ti, Cu, stainless-steel and tungsten-copper (WCu). The neutron yield with the WCu electrode was maximum—50% more than that obtained with the Cu electrode.

Nanosecond rise time air-core current transformer for long-pulse current measurement in pulsed power

A slow-wave delay line type air-core (nonmagnetic Nylon former) current transformer fabricated using silver epoxy for the measurement of currents of long pulse durations and few nanoseconds rise times is reported in this article. The advantage of using silver epoxy is that it fills all the voids between coil and shield and enhances the proximity of the coil to the shield, leading to a high value of distributed capacitance. Thus the transit time of the coil increases and it can measure fast current pulses of longer durations. Increasing the inductance of the coil can compensate for the resulting reduction in the sensitivity of the coil for matched termination. An easy experimental technique to find the value of the matched terminating resistor is also reported in this article. We have also done simulations of the slow wave current transformer using PSPICE.